Audio coding is used in many applications and environments such as satellite radio, digital radio, internet streaming (web radio), digital music players, wireless mobile devices, and a variety of mobile multimedia applications. There are many audio coding standards, such as standards according to the motion pictures expert group (MPEG) including MPEG-2 and MPEG-4, standards according to the International Telecommunications Union (ITU), windows media audio (WMA) standards, and standards by Dolby Laboratories, Inc. Many audio coding standards also exist, including the MP3 standard and successors to the MP3 standard, such as the advanced audio coding (AAC) standard used in “iPod” devices sold by Apple Computer, Inc. Audio coding standards generally seek to achieve low bitrate, high quality audio coding using compression. Some audio coding is “loss-less,” meaning that the coding does not degrade the audio signal, while other audio coding may introduce some loss in order to achieve additional compression.
In many applications, audio coding is used along with video coding in order to provide multi-media content for applications such as video telephony (VT) or streaming video. Audio coding is also commonly used in mobile devices that support multimedia applications such as video games, and the like. Indeed, mobile devices increasingly incorporate a wide variety of functionality and content that provides audio output to a user, and therefore makes use of audio coding techniques. Audio and video information may be segmented into frames or packets, which comprise blocks of audio and video data. A stream of audio output can be transmitted as a sequence of audio frames or packets, and then decoded to provide audio output to users.
Audio coding is also used in audio broadcasting or multimedia broadcasting. Multimedia broadcasting, for example, may use Enhanced H.264 video coding for delivering real-time video services in terrestrial mobile multimedia multicast (TM3) systems. Such TM3 systems may use the Forward Link Only (FLO) Air Interface Specification, “Forward Link Only Air Interface Specification for Terrestrial Mobile Multimedia Multicast,” to be published as Technical Standard TIA-1099 (the “FLO Specification”). Other types of multimedia broadcasting standards and techniques also exist including future standards and techniques that may emerge or evolve.
Many audio streams carry clock signals from the encoder. In this case, the encoder provides clock signals in the audio stream that can be used by the decoder to ensure that timing of audio output is correct. The clock signals, for example, may be used to adjust a local clock at the decoder so that the decoder is clocked at the same rate as the encoder, thereby avoiding problems or errors in the timing of audio output. Audio streams that carry clock signals include audio packet streams that use the MPEG-2 Systems Transport Stream defined e.g., in ISO/IEC 138181-1. Many other examples of audio streams that carry clock signals may also exist, including future techniques that may emerge or evolve.